Arguably, nothing in the world impacts the daily lives of people more than the weather.
Perhaps nowhere is the effect of weather more strongly felt than on the runways of world's airports. Haze, mist, fog, and rain can and do prevent airplanes from being able to take off and land. Not only do these weather conditions present an inconvenience to travelers in the form travel delays, these conditions can be life threatening when fog, for example, makes it impossible for an airline pilot to see the runway. Naturally, such weather conditions also impact life in countless other ways as well.
Because of these effects, researchers have attempted to build machines and develop models that can accurately gauge the weather. For example, at many airports, visibility meters have been put in place for determining the visibility at discrete locations around the grounds of the airport. Such machines detect the impact of atmospheric conditions (such as fog) on a beam of light transmitted from a light source, such as a laser, to a detector. These machines rely on the principle that particles in the atmosphere scatter light incident on those particles. Such machines are problematic, however, in that they only sample light in a small region (e.g., a two foot by two foot region) and are based on single scattering. Moreover, because of the complexity of these machines, they are quite expensive.
In contrast to the single scattering principle used in these machines is the principle of multiple scattering. One easily understood illustration of multiple scattering is the manner in which bad weather impacts the appearance of light sources, especially when viewed at night. This impact is typically seen in the form of a glow or halo that appears to surround a light source. The appearance of this glow or halo is the result of light rays being deflected multiple times by particles in the atmosphere as they leave the light source, such that the rays appear to be originating from an area surrounding the light source.
Attempts have been made to model the effects of multiple scattering on light rays in the atmosphere. For example, one approach has been to assume that the paths of light traversal are random and then to apply numerical Monte-Carlo techniques for ray tracing. Monte-Carlo techniques are problematic, however, in that they attempt to predict the paths of light based upon each photon impacting each particle in the medium. Such an approach is highly dependent upon the specific properties of the medium, such as the density of the medium, type of particles in the medium, the size of the medium, etc. As a consequence of this, millions of rays must be traced through the atmosphere to accurately model the multiple scattering using Monte-Carlo techniques. Thus, when attempting to model a single light source in a single image, this technique can take several hours using current computer systems. Obviously, such a long processing time is inadequate for real-time or near-real-time image processing.
Monte-Carlo techniques are also problematic in that they do not converge for pure scattering media. This may be unacceptable when trying to model multiple scattering of certain media.
Another technique for modeling the impact of particles in the atmosphere on light is to use the plane parallel model. This model has been used in fields such as atmospheric optics and astronomy in which the medium being observed (e.g., the atmosphere around the earth) is illuminated by a distant light source such as the sun or moon. The plane parallel technique is problematic, however, in that it assumes that the light source is entering the medium from an infinitely far away distance, which is not the case when modeling the impact of an atmosphere on the appearance of a light source that is inside that atmosphere.
Accordingly, other techniques for modeling the multiple scattering of light from encompassed light sources and for using those models are desired.